This invention relates to solid state optical waveguide structures.
Optical waveguide structures based on silicon dioxide are used to prepare a variety of integrated optics devices. A typical waveguide structure includes a silicon (Si) substrate having three layers of silicon dioxide (SiO2), each doped to produce a desired index of refraction and reflow properties. The first layer on the silicon substrate, called the lower cladding layer, typically comprises 10-15 microns of thermally grown, undoped SiO2 formed by high-pressure oxidation of the silicon substrate surface (HIPOX). The second SiO2 layer, called the core layer, is doped with phosphorus (P) and having an index of refraction larger than that of the lower cladding layer. The third, or upper layer, overlies the core layer, has a thickness also in the range of 10-15 microns, and is typically doped with Boron (B) and Phosphorous to have an index of refraction close to that of the lower cladding layer. A higher refractive index of the core relative to the cladding layers is required to support waveguiding conditions for propagation of light in the core.
One of the problems encountered in the fabrication of photonic lightwave circuits (PLCs) based on waveguides is optical birefringence caused by biaxial (vertical and horizontal) strain exerted by the cladding layers on the core. The strain arises from the difference in thermal expansion coefficients of the cladding layers and core and the Si substrate. The result is anisotropic index of refraction, i.e. different indices of refraction for the TE and TM modes of optical waveguides. In practical multiplexing devices this difference may shift the response spectrum between the TE and TM modes of individual channels by as much as 0.3 nm. This effect, known as birefringence, can be very significant in dense wavelength division multiplexing (WDM) where the channel separation may be smaller than 0.4 to 0.8 nm.
In typical PLC structures, cladding layers comprise layers having low level boron dopant concentrations. For example, U.S. Pat. No. 5,506,925 discloses a cladding layer doped with 2.5% P and 4% B to produce a desired index of refraction and also proposes irradiation to induce a reduction in birefringence in the waveguide structure. Without such irradiation, thermally induced stress in the waveguide structure core may be as high as xe2x88x9275 MPa, resulting in birefringence induced splitting between TE and TM modes of 0.25-0.3 nm which in some commercial applications is undesirably high.
In PLC structures using such B and P doped cladding layers, the as-deposited doped upper cladding layer is not yet glass of high optical quality. Formation of glass requires annealing the doped cladding layer at high temperature, typically between 900 and 950xc2x0 C. The softening temperature for undoped silicon dioxide is considerably higher, as much as 1100xc2x0 C. As the annealing temperature is approached the doped silicon dioxide softens and the strain in the core layer becomes negligibly small. However, upon cooling the glass becomes rigid again. The difference in thermal expansion coefficients between the glass and the Si substrate gives rise to strain and, therefore, undesirable optical birefringence.
The present invention seeks to address and alleviate the above problems without use of irradiation.
According to the present invention, an optical waveguide structure comprises an optical waveguide core surrounded by lower and upper cladding layers of silicon dioxide on a silicon substrate. The core comprises phosphorous doped silicon dioxide, advantageously having a stoichiometric composition to reduce optical loss. The respective refractive indices of the core, the lower cladding layer and the upper cladding layer enable waveguiding of optical signals introduced into said core. The upper cladding layer, and in one embodiment each of the upper and lower cladding layers, comprises silicon dioxide doped with phosphorous and about 9% or greater of boron such that birefringence induced optical shift between TE and TM modes of propagation of an optical signal in said core layer does not exceed about 0.15 nm, and preferably is reduced to below about 0.06 nm.
Desirably, the thermal coefficient of expansion of each of said upper and lower claddings layer approximates that of said silicon substrate. Advantageously, the waveguide structure is so formed that there is close to zero residual stress in the waveguide core. To address this objective, the dopant relationships in the cladding around the core are such that residual tensile stress at least partly compensates compressive strain in the core. Alternatively, the lower cladding layer may comprise a HIPOX layer, preferably having a pedestal on which the waveguide core is positioned so that the upper cladding layer surrounds the top and side surfaces of the core and extends beyond the lower surface of the core along the side surfaces of the HIPOX layer pedestal. In such a structure, the upper cladding layer can be structured to provide a desired index of refraction (e.g. 1.446) and to have a residual tensile strain (e.g. about 8 Mpa) to compensate compressive stress in the core and enabling birefringence as low as about 0.01 nm to be obtained.
In general, waveguide structures embodying the invention may include a core comprising annealed phosphorous doped silicon dioxide having a phosphorous content of about 8%, in conjunction with at least an upper cladding layer comprising annealed silicon dioxide doped with boron in the approximate range 9% to 11% and with phosphorous in the approximate range 2.5% to 3.5%. In a particular embodiment, the core and cladding layers are formed using low temperature PECVD TEOS processing.